A two-wire transmitter is a device, which senses a physical quantity such as a flow rate or a pressure and then outputs the sensed value (hereinafter, referred to as “sensor signal”). As a two-wire transmitter that converts a sensor signal to an analog voltage signal to be output, for example, there is known a two-wire transmitter described in Patent Document 1.
FIG. 5 is a diagram for discussing a conventional technique of the two-wire transmitter. Such an illustrated two-wire transmitter 10 is provided with: an operational amplifier 1 for amplifying sensor signals M1 and M2 output from a sensor S; a switch 3 capable of switching a connection target in accordance with the voltage value of a sensor signal Vsns output from the operational amplifier 1; and resistance elements 5, 6, and 7 connected to the switch 3. The two-wire transmitter 10 is also provided with: a reference voltage generator 4; a resistance element 6 connected to the switch 3; and an operational amplifier 2 that changes an output current Iout depending on the resistance magnitude of the resistance element 7.
The sensor S is a sensor for sensing the physical quantity such as a magnetic strength, temperature, pressure, or the like.
The two-wire transmitter 10 illustrated in FIG. 5 operates as will be described below.
The operational amplifier 1 outputs the sensor signal Vsns that continuously changes. When the sensor signal Vsns exceeds a prescribed threshold value, the connection target of the switch 3 is switched from the resistance element 6 to the resistance element 7. The potential difference between the output signal Vout of the operational amplifier 2 and the ground (GND) is divided by the resistance element 5, and the resistance element 6 or the resistance element 7 connected to the switch 3. The signal in accordance with the divided potential is input into the operational amplifier 2, as a feedback signal Vfb. The operational amplifier 2 operates to equalize the reference voltage Vref generated by the reference voltage generator 4 and the voltage value of the feedback signal Vfb.
For this reason, when the voltage value of the feedback signal Vfb increases, the operational amplifier 2 operates to increase the output current Iout. In this situation, the voltage value of the output signal Vout decreases to equalize the reference voltage Vref and the voltage value of the feedback signal Vfb.
On the other hand, when the voltage value of the feedback signal Vfb decreases, the operational amplifier 2 operates to decrease the output current Iout. As a result, the voltage value of the output signal Vout from the operational amplifier 2 increases to equalize the reference voltage Vref and the voltage value of the feedback signal Vfb.
FIG. 6 is a diagram showing a relationship between the sensor signal and the output signal in the conventional two-wire transmission lines. In FIG. 6, the vertical axis represents the voltage value of the output signal Vout, and the horizontal axis represents a resistance R of the two-wire transmitter. The relationship between the resistance R and the voltage value of the output signal Vout of the two-wire transmitter is expressed by the following equation (1). In the equation (1), R0 is a resistance value of the resistance element 5 shown, and R is any one of resistance values (R1 and R2) of the resistance element 6 and the resistance element 7, to be chosen by the switch 3.Vout=Vref+(R0/R)·Vref  (1)
The sensor signal Vsns output from the sensor S continuously changes. Then, when the sensor signal Vsns exceeds a prescribed threshold value, the connection target of the switch 3 is switched to the resistance element 7 from the resistance element 6.
Prior Art Documents
Patent Documents
Patent Document 1: U.S. Pat. No. 6,437,581 B1